You don’t need to watch the coaches’ film to know that something’s up with the Patriots defense. The unit hasn’t played well at all in recent weeks, and journeyman Jason Campbell lit them up for 494 total yards (386 in the air) in
New England’s 27-26 win over the Browns Sunday.

The defense was the backbone of the team through the first six weeks of the season. So what has happened? The team won’t come out and tell us, of course, but after reviewing the coaches film from the last several weeks, we have a theory.

The crux of the matter is injuries, of course. Players such as Chris Jones, Joe Vellano, Sealver Siliga and Dane Fletcher have filled in admirably for Vince Wilfork, Tommy Kelly, and Jerod Mayo, but those rookies and role players can’t possibly sustain an elite level of play over the course of a season.

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But the Patriots’ woes go beyond that.

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One thing you notice is that the Patriots, who began the season almost strictly as a press-man coverage team, have played a lot more zone in recent weeks — mostly cover 2 and cover 3. Why the switch? The secondary is all banged up right now — Aqib Talib slowed by a hip injury, Alfonzo Dennard by a meniscus injury, Kyle Arrington with a groin ailment, and all kinds of nicks and bruises for other players that don’t show up on the injury report.

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Playing zone can mask the injuries — it’s a lot easier to cover a zone than it is to chase a receiver all over the field in one-on-one coverage.

Playing zone is also a lot more complicated than playing man-to-man defense. It takes a lot of communication, and on several occasions against both the Browns and Texans, the Patriots had clear communication breakdowns and allowed receivers to run free across the middle.

Andre Johnson ate the Patriots alive two weeks ago, and Josh Gordon and Jordan Cameron did it Sunday, moving the chains repeatedly with crossing routes. The Patriots haven’t given up too many big plays over the top with their zone — Cleveland’s two long plays were both short catches and long runs — but they’re getting chewed up in the short and medium portions of the defense.

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It also doesn’t help that the Patriots are hardly getting any pressure on the quarterback, with just six sacks in their last four games. They have absolutely zero interior pass rush from their rookie defensive tackles, they don’t blitz much (we counted 11 in 47 passing plays against Cleveland) and Rob Ninkovich and Chandler Jones, who have each played over 95 percent of snaps this year, seem to be running out of gas. Campbell had all day to sit in the pocket, diagnose the zone defense, and pick it apart.

Add it all up — backups subbing for Pro Bowlers, defensive backs struggling through injuries and having too many communication errors, and the front four not getting to the passer — and you have a defense that has allowed 414 yards and 28.6 points over their last five games.

Other observations from the victory over the Browns after reviewing the tape:

When the Patriots had the ball . . .

■ The Browns also played a lot of zone coverage and didn’t blitz much, but their front four did a much better job of harassing Tom Brady. Jabaal Sheard (1 sack) got consistent pressure against Nate Solder, while Barkevious Mingo (1 sack) and Paul Kruger (2 sacks) took advantage of Will Svitek at right tackle. The best block may have come from Danny Amendola, who pancaked linebacker D’Qwell Jackson on Shane Vereen’s touchdown run.

The Browns’ problem was that they played it too safe in the final three minutes. We counted 11 blitzes in the Patriots’ first 44 passing plays, but the Browns didn’t blitz once on the final two drives, which included 12 throws. The Browns played soft zone defense to protect the end zone, and with the defensive linemen tired, Brady had all day to sit in the pocket and find Vereen and Julian Edelman wide open underneath. Brady was 10 for 12 on his final two drives, and one of the incompletions was a spike.

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■ The Patriots clearly liked their matchups against linebackers Jackson, Mingo, and Craig Robinson. They hit a 31-yard wheel route to fullback James Develin against Mingo, hit a 21-yard seam pass to Rob Gronkowski, a 32-yard checkdown to LeGarrette Blount against Jackson, and Vereen absolutely abused Robinson, burning him for a 50-yard reception and then a 16-yarder on consecutive plays in the third quarter.

It was interesting to see the Browns double-team Vereen, and not Gronkowski, early in the third quarter.

■ Also interesting that the Patriots scored all 27 of their points, and gained 293 yards, after Gronkowski left the game. Matthew Mulligan essentially filled Gronk’s spot in the third quarter, but was mostly used as a sixth blocker. It gave Brady fewer options, but the Browns also didn’t expect Mulligan to run a slip screen for 15 yards, either.

Inside the final three minutes, though, the Patriots strictly used four wide receivers and Vereen.

■ Blount had another great game and is really coming on strong. He had runs of 9 and 7 yards after contact, and on his 31-yard catch, he slipped three defenders and plowed through two more before being dragged out of bounds.

When the Browns had the ball . . .

■ The pass rush is nonexistent, but Patriots switched back to mostly a 4-3 base defense this game, and the front four did well against the run, particularly Siliga, who played more than Vellano and Isaac Sopoaga for the first time this season and more than held his own. Take out a 34-yard end-around for Gordon and a 21-yard scramble for Campbell, and the Patriots held the Browns to 53 rushing yards on 23 carries (2.3 average).

■ The Patriots paid dearly the few times they blitzed. Gordon’s 80-yard catch-and-run came against a seven-man blitz and left Talib completely on an island with Gordon, who is obviously faster and more physical. They also blitzed six on Gary Barnidge’s 40-yard catch-and-run. Talib totally gave up on the play when he saw the penalty flag fly, but there was also a major coverage error on the play. Barnidge was wide open, while Devin McCourty stood around with no one to cover.

■ Cameron caught all nine passes thrown his way for 121 yards and a touchdown. Three of those receptions came against Steve Gregory, and two each against Jamie Collins and Logan Ryan. Collins bit badly on Campbell’s play fakes in the fourth quarter, first letting Cameron run wide open for a 16-yard catch and then again on Cameron’s 4-yard touchdown catch.

Special teams . . .

■ Stephen Gostkowski is 30 of 32 on field goals this year, including 5 of 6 from 50-plus yards, but his most impressive kick of the season went slightly less than 10 yards. He executed a perfect “bunt” on the onside kick, and to do it while approaching the football at close to full speed shows incredible athleticism. His 50-yard field goal in the cold wasn’t too shabby, either.

■ Josh Boyce showed great moves in the open field, avoiding four tackles on a slip screen, but he needs to stop taking the ball out of the end zone so often on kickoffs. He came out from 4 yards deep and was tackled on the 14.

■ Just a dumb, dumb, dumb decision by Browns coach Rob Chudzinski to go for 2 with 10:15 to go in the third quarter. If he had just kicked, the score could have been 20-10 in the fourth quarter, instead of 19-11. Never go for 2 until you absolutely have to.

Game balls . . .

■ Brady: 16 of 20 for 158 yards and two touchdown passes in the fourth quarter.

■ Vereen: Nine of his 12 receptions came after Gronkowski left the game, for 126 of his 153 receiving yards.

■ Edelman: Brady didn’t bother looking at any of the other receivers on the second-to-last drive.

■ Donta Hightower: The linebacker had his most active game of the season and finished with a team-high 11 tackles, including several impressive stops in the run game.